The invention relates to a device for detecting faults and/or measuring the wall thickness of continuously moving strips, sections or tubes of plastics, using ultrasonic signals, which, for the complete coverage of the wall of the strip, section or tube, are introduced perpendicularly into the plastics from measuring heads distributed one beside the other and transversely of the direction in which the strip, section or tube runs, over the width of the strip or section or the periphery of the tube, and fixed above the strip, section or tube, their reflected signals being received by them and supplied to an evaluation unit.
In a known device of the kind specified (U.S. Pat. No. 4,740,146) for cylindrical tubes a few measuring heads are disposed fixed with a relatively large spacing around the tube, thus covering the tube wall at only a few strips extending in the direction in which the tubes move. Such a device for wall thickness measurement may, as a rule, be adequate since variations in thickness over the periphery are usually not limited to narrow longitudinal strips but extend over a fairly large peripheral area. However, if variations in thickness deviate from the rule, no fault-free wall thickness measurement is possible. Such a device is basically unsuitable for the detection of faults, since faults are frequently confined exclusively to a very small place.
However, the tube wall can be checked over its entire periphery using another known device (DE 40 33 443 A1) which has a measuring head extending over the entire periphery of the tube. However, driving and guiding such a measuring head involves considerable costs for machinery, and there is also the fact that such a measuring head can cover the continuously moving tube only over spiral paths. In this case also, therefore, there may be zones which are not covered.
To enable a tube to be covered over its entire surface, measuring heads might form a closed ring. However, one disadvantage of such a device is that it necessitates a large number of measuring heads.
To enable faults and their orientation to be detected in a tube wall, a device for fault detection is known (U.S. Pat. No. 4,523,468) which uses two groups of measuring heads disposed one behind the other on the external periphery of the tube to introduce ultrasonic signals into the tube wall and receive signals reflected at faults. To the end the first group generates signals which spread out axially in the tube wall, while the second group generates ultrasonic signals spreading out in the peripheral direction. Allowing for the position of the measuring head emitting an ultrasonic signal and the position of the measuring head receiving the reflected signal and also the propagation time, it is possible to determine the position of the fault in the tube wall. The position and orientation of the fault can be determined by a number of such measurements. To prevent the mixing of the ultrasonic signals emitted by the individual measuring heads in the receiving measuring heads, the measuring heads are so operated in succession by a multiplex method that a number of receiving measuring heads are associated with each measuring head emitting an ultrasonic signal.
The wall thickness of tubes and the like cannot be determined using such a device fault direction, in which the measuring heads of one group transmit exclusively axially ultrasonic signals, the other group of measuring heads extending exclusively peripherally of the tube and emitting ultrasonic signals. Since this device also involves exclusively the determination of relatively large faults and their orientation in the tube, the problem of covering the tube over its whole area using solely measuring heads operative in a relatively small zone plays no part.
It is an object of the invention to provide a device for the complete coverage of the wall thickness and/or faults in strips, sections or tubes with a convexly curved surface.
This problem is solved according to the invention in a device of the kind specified by the features that the measuring heads are so disposed one beside the other and transversely of the direction in which the strip, section or tube moves, allowing for the sonic propagation, scatter and refraction of the reflected signals on the strip, section or tube, that they completely cover the wall of the strip, section or tube, those signals of a signal emitted by the transmitter of each measuring head which are reflected on the strip, section or tube being received by said measuring head and by the measuring heads adjacent on each side thereof; while the number of measuring heads is   N  ≥            π      ⁢              xe2x80x83            ⁢      R              S      ⁢              xe2x80x83            ⁢      tan      ⁢              xe2x80x83            ⁢      α      
The diameter of the receiving of each measuring head is   K  ≥            2      ⁢              xe2x80x83            ⁢      π      ⁢              xe2x80x83            ⁢      R        NT  
And the wavelength of the ultrasonic signal is   λ  ≥            R      ⁢              xe2x80x83            ⁢      1        -          cos      ⁢              360        NT            
where
N=number of measuring heads;
R=external radius of curvature of the surface of the strip, section or tube;
S=distance of the measuring head from the curved surface;
xcex1=opening angle of the ultrasonic transmitter of the measuring heads;
K=diameter of the receiving surface of each measuring head and
T=number of measuring zones per measuring head;
xcex=wavelength of the ultrasonic signal.
While in conventional devices for fault detection and/or wall thickness measurement using measuring heads operating by the ultrasonic pulse echo method, the receiver of a measuring head receives exclusively the signal emitted by the transmitter of said measuring head and reflected, but not the signals refracted at the curved surface and therefore xe2x80x9clostxe2x80x9d to the measurement, according to the invention even these xe2x80x9clostxe2x80x9d portions of the emitted and reflected ultrasonic signal are utilized for the measurement, said signals being received by the sensors of the adjacent measuring heads. This enables the entire surface of the object to be measured to be covered completely using a few ultrasonic measuring heads. Since as a result the signal emitted by an ultrasonic measuring head passes over a larger area, fewer measuring heads are required than in the conventional devices.
If the rules according to the invention for the design of the measuring heads are respected, telescoped bell-shaped curves for the sonic pressure of adjacent measuring heads are obtained for the ultrasonic signals emitted from the ultrasonic measuring heads and reflected on the tube or strip to be tested. Due to the design of the measuring heads, said bell-shaped curves must be telescoped in one another to such an extent that the sonic pressure for the evaluation of echo signals is still adequate as far as the intersection point of the bell-shaped curves. When determining the number of measuring zones per measuring heads, it must be remembered that measuring zones are partially xe2x80x9cmirroredxe2x80x9d, since between two adjacent measuring heads the same measuring zone is first covered by the reflected signal emitted by one measuring head and received by the other adjacent measuring head, and then by the reflected signal transmitted from the other measuring head and received by an adjacent measuring head. Such xe2x80x9cmirroredxe2x80x9d measuring zonesxe2x80x94i.e., zones covered twice, count only once.
It is basically possible for the surface to be covered without any overlapping, taking into account the unrefracted, reflected signals and the refracted, reflected signals. In that case, the smallest number of measuring heads per unit of area measured will suffice. Provision cannot however also be made for overlapping to take place, so that each portion is investigated not only using a reflected signal without refraction, but also using a refracted reflected signal. In the latter case there is the advantage that due to their different angle of incidence, the refracted signals can detect faults which can hardly be detected by an unrefracted signal.
To facilitate the processing of the signals, provision can be made that not all the signals are supplied in parallel to the valuation unit, but the measuring heads are operated cyclically. However, a number of cycles can run in parallel.